BIL is a power industry term.  Quoting from 'Standard Handbook for Eletrical
Engineers', Fink & Beatty:

(P 17-20, para 25, in my copy...)

	... in an attempt to arrive at some level of standardization of 
	practices, minimum electrical spacings for standard Basic Insulation
	Levels (my emphasis...dwp) were ... published in AIEE paper 54-80..

Accompanying table covers 7.5KV "class", with a reccomended BIL of 95kv
WITHSTAND  up to 1175KV BIL, with no listed 'class'.  Footnotes point out that
this changes wiht altitude, etc.

Other readings indicate that this is intended to provide a guide in picking
surge/lightning arresters: if the system nominal is 7.5KV and all components
are 95KV BIL tested/designed then arrestors et for (o, say) 50-75KV should
keep everything happy.  If everything is 7.5KV nominal, but NOT otherwise
specified, one piece might survive to 150kv, another fail at 10 KV and the
lightning arrestors would need to be picled to protect the most vulnerable

Lightning is ONE source of an impulse.  Just as the coiler sees surges closing
on deenergized equipment, when the power co closes on a 2 MW transformer from a
10 KV source THEY see some NASTY surges travelling down the lines.  These
switching surges can be every bit as tricky as lightning.